So-called Type I and Type II hot corrosion can be controlled by adding chromium to the surface of a component by a process known as chromising. This produces a diffused chromium barrier on the component substrate. U.S. Pat. No. 6,283,715, published on 4 Sep. 2001, describes the use of such a diffused chromium layer on the surface of a turbine blade, and specifically on a part of the blade below the so-called platform and on an aerofoil part above the platform.
While the use of a diffused chromium layer by itself offers good protection against Type II corrosion, protection against the higher-temperature Type I corrosion usually requires the addition of diffused aluminum, which results in a chrome-modified aluminide coating on the substrate surface. Such a constitution is also described in U.S. Pat. No. 6,283,715, the aluminum diffusion layer being applied to the chromium diffusion layer in the aerofoil region above the platform. This patent also describes the application of an overlay layer in the form of a ceramic coating, which serves as a thermal barrier coating to insulate the underlying layers. This overlay is applied to the aerofoil part of the blade only.
U.S. Pat. No. 6,270,318, published on 7 Aug. 2001, describes a turbine blade, in which the area between the platform and the root is coated with a ceramic overlay. U.S. Pat. No. 6,296,447, published on 2 Oct. 2001, discloses a turbine blade having an upper platform surface which is coated with a first layer, which may be a diffusion aluminide layer, followed by a second, ceramic layer. A similar coating arrangement is employed on the pressure side of the aerofoil of the blade in question.